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Abstract:

A height-finding 3D avian radar comprises an azimuthally scanning radar
system with means of varying the elevation pointing angle of the antenna.
The elevation angle can be varied by employing either an antenna with
multiple beams, or an elevation scanner, or two radars pointed at
different elevations. Heights of birds are determined by analyzing the
received echo returns from detected bird targets illuminated with the
different elevation pointing angles.

Claims:

1. A 3D radar system comprising:an antenna provided with means for varying
its effective pointing direction in elevation;a radar transmitter
operatively connected to said antenna for generating a radar signal for
emission via said antenna;a radar receiver operatively connected to said
antenna;an azimuth scanner operatively coupled to said antenna for
rotating same about an axis; anda processor operatively connected to said
receiver, said processor being configured for detecting and localizing
airborne targets in azimuth and range, said processor being further
configured for estimating a height of each detected target based on
relative amplitudes of echo returns as a function of elevation pointing
direction of said antenna.

2. The system defined in claim 1 wherein said means for varying the
effective pointing direction of said antenna in elevation includes means
for generating at least two beams via said antenna and means for
selecting a given beam for a given radar pulse.

3. The system defined in claim 2 wherein said means for generating and
said means for selecting includes a high-power RF switch that rotates
with said antenna about said axis, said switch being operable on both
transmit and receive.

4. The system defined in claim 2 wherein said means for selecting includes
a low-power RF switch that is stationary relative to said axis and does
not rotate with said antenna, said switch being operable on receive only.

5. The system defined in claim 2 wherein said antenna is a reflector, said
means for generating including multiple feeds.

6. The system defined in claim 2 wherein said antenna is a
frequency-scanning antenna and said means for generating includes a
variable-frequency transceiver tunable to generate said at least two
beams.

7. The system defined in claim 2 wherein said antenna is a phased-array
antenna and said means for generating and said means for selecting
include a beam forming network.

8. The system defined in claim 1 wherein said antenna is an elevation
monopulse antenna and said radar receiver includes a dedicated receiver
for each antenna receive channel.

9. The system defined in claim 8 wherein said antenna is a reflector with
multiple feeds.

10. The system defined in claim 8 wherein the dedicated receivers are
non-coherent and the associated receive channels are from the upper and
lower beams of said antenna.

11. The system defined in claim 1 wherein said radar transmitter and said
radar receiver are noncoherent.

12. The system defined in claim 1 wherein said radar transmitter and said
radar receiver are from a COTS marine radar.

13. The system defined in claim 1 wherein said receiver has a digitized
output.

14. The system defined in claim 1 wherein said means for generating
includes an elevation scanner.

15. The system defined in claim 1 wherein said processor is a COTS PC.

16. The system defined in claim 1 where said processor executes
integration, interference suppression, clutter suppression, and adaptive
thresholding.

17. A method of determining the heights of airborne targets,
comprising:operating a radar system to illuminate and detect the targets,
said radar system having at least one radar antenna;during the operating
of said radar system, varying an antenna elevation pointing angle of said
at least one radar antenna; andestimating detected target heights in
accordance with variation in amplitude of echo returns as a function of
antenna elevation pointing angle.

18. The method defined in claim 17 wherein said varying comprises emitting
a plurality of beams via said antenna.

19. The method defined in claim 18 wherein radar transmission and
reception is alternated between said beams from pulse to pulse.

20. The method defined in claim 18 wherein radar transmission and
reception is through all of said beams on every transmission pulse.

22. The method defined in claim 17 wherein said radar system includes at
least two radar subsystems proximate to one another, said varying
comprising operating said at least two radar subsystems so that each
radar subsystem is pointed at a different elevation angle.

23. The method defined in claim 17, further comprising operating a
processor to track the detected airborne targets.

24. The method defined in claim 17 wherein said estimating includes
interpolating in elevation.

25. The method defined in claim 17 wherein said estimating includes using
target tracks in an association process to identify tracks in different
beams belonging to a common target, thereby enabling a smoothing and
improving of height estimates.

26. The method defined in claim 17, further comprising using the height
estimates further to estimate target radar cross-section.

27. The method defined in claim 17, further comprising using the height
estimates further for target classification.

28. The method defined in claim 17, further comprising distributing height
information to a network.

29. The method defined in claim 17, further comprising automatically
notifying or alerting users of hazards or situations of interest.

30. The method defined in claim 17, further comprising combining target
height and range estimates and radar echo intensities to form accurate
estimates of radar cross-sections.

31. The method defined in claim 17, further comprising continually
updating estimated dynamics vectors, including speed, heading, position,
height, and respective uncertainties thereof for each of said targets.

32. The method defined in claim 17 wherein said estimating is carried out
for all targets in a field of view of said radar system simultaneously,
to provide 3D localization of such targets.

Description:

FIELD OF THE INVENTION

[0001]This invention relates to ground-based radar systems and methods.
The invention relates more additionally and more specifically to radar
target detection, tracking and estimation of target height. The invention
is particularly useful in radar surveillance of birds and other airborne
targets.

BACKGROUND OF THE INVENTION

[0002]Avian radars are used to track birds in flight in the vicinity of
airfields, wind farms, communications towers, and along migration routes.
Birds are a significant hazard to aviation safety. Applications that
require bird monitoring are the bird aircraft strike hazard (BASH)
problem and the natural resource management (NRM) problem. Billions of
dollars in damage to aircraft and significant loss of life have been
recorded due to birds flying into aircraft, particularly during take-off
and landing in the vicinity of airports.

[0003]The danger associated with birds depends on their altitude (among
other factors). Users of bird detection and tracking radars need to know
the height of tracked birds. State-of-the-art avian radars provide target
tracking with localization in only two dimensions. These systems do not
estimate height (within the beam extent) in any real sense. Thus avian
radars need altitude estimation of bird (or other airborne) targets. They
need the means to estimate target height in a manner that is practical
and economical. The purpose of the current invention is to provide next
generation avian radars with such means, thereby overcoming current
limitations in the state-of-the-art.

[0004]State-of-the-art avian radars use inexpensive,
commercial-off-the-shelf (COTS) X-band marine radar transceivers, fitted
with slotted-waveguide array antennas, as well as parabolic reflector or
Cassegrain (dish) antennas. The raw received baseband signals are
digitized, followed by detection and tracking of bird targets.
State-of-the-art avian radars provide continuous, day or night,
all-weather, situational awareness with automated detection, localization
and warnings of hazards. They provide high-quality target track data with
sophisticated criteria to determine potentially dangerous target
behavior, as well as communication of alerts to users who require that
information. They also minimize operator interaction.

[0012]COTS marine radars are very inexpensive. These marine radars exhibit
surprisingly good hardware specifications. However, as-is, these radars
deliver mediocre performance for bird targets because of their primitive
signal processing. Combining a COTS marine radar with a digitizer board
and a software radar processor that runs on a COTS personal computer (PC)
and a parabolic dish antenna forms a state-of-the-art avian radar, one
with a very limited three-dimensional (3D) localization capability.
Modifying such radars via custom antennas and processing allows height
estimation and coverage.

[0013]Slotted-waveguide array antennas are used to provide two-dimensional
(2D) localization (i.e. range and azimuth, which can be translated to
latitude and longitude). These systems provide good volume coverage due
to the typically larger vertical (elevation) beamwidth, which is on the
order of 20 degrees. Such systems cannot provide useful height estimates
of tracked targets when the radar is spinning horizontally in its usual
orientation. This is because the beam uncertainty in the 3rd
dimension (elevation), which is on the order of the beam extent, is too
large. For example, the elevation beam extent or height uncertainty for a
target at a distance of just 1 km from the radar is about 1,000 feet.
This means that if both a plane and a bird are being tracked by the radar
at a distance of 1 km away, the radar cannot tell whether the two targets
are 1,000 feet apart (i.e. one is on the ground and the other is at the
upper edge of the vertical beam, 1,000 feet off the ground) or whether
they are at the same altitude where a collision could occur. While some
radar configurations orient the slotted-array antenna so that it spins
vertically (rather than horizontally) to get a measure of height, see
Nocturnal Bird Migration over an Appalachian Ridge at a Proposed Wind
Power Project, Mabee et al, Wildlife Society Bulletin 34(3), 2006, page
683, they still can only operate as 2D radars. In order to measure
height, they can no longer provide 360-degree azimuthal coverage (which a
conventional azimuth-rotating radar provides).

[0014]Parabolic reflector or Cassegrain (dish) antennas are used today to
provide a very limited 3D localization capability. These antennas employ
a single beam (pencil shaped), fixed in elevation, but rotating in
azimuth. The azimuth rotation results in the usual 2D, 360-degree
coverage with localization in range-azimuth or latitude-longitude.
However, by using a narrow pencil beam (say between 2 and 4 degrees
wide), the height uncertainty reduces significantly as compared to the 20
deg slotted-array antenna. Using the previous example, with targets at a
distance of 1 km from the radar and a 4-degree dish antenna, height
estimates with uncertainties on the order of 200 feet are now possible.
While providing useful height information at very short ranges, the
height estimates are still of limited use at further ranges. Also, volume
coverage is restricted accordingly with the narrower pencil beam. The
present invention seeks to overcome these limitations by providing better
3D localization capabilities. In particular, means are disclosed herein
to provide both better height estimates (reduced height uncertainty) and
greater volume coverage.

[0015]Merrill I. Skolnik in his Introduction to Radar Systems, 2nd
Edition, McGraw-Hill Book Company 1980 and his Radar Handbook, 2nd
Edition, McGraw-Hill, Inc., 1990, describes height-finding radars that
use nodding horizontal fan beams. These radars are steered to the bearing
where targets have been detected by an independent 2D air-surveillance
radar. These height-finding radars can not get height estimates for more
than 20 or so targets per minute, and have problems with
azimuth-elevation (Az-El) ambiguities in dense target environments.
Military airborne and land-based tracking radars provide height
information for a single target only (via closed-loop steering in both
dimensions). They use monopulse or sequential lobing techniques to obtain
the off-boresight error signals, but like the height-finding radars, are
unable to perform 3D surveillance. Military 3D surveillance radars, on
the other hand, employ rotating phased array antennas that form either
multiple receive beams or rapidly electronic-scanning pencil beams. See
Radar Applications, Merrill I. Skolnik, IEEE Press New York, 1987. Like
these radar systems, the present invention is also true 3D surveillance;
its antenna rotates in azimuth while estimating height. However, the
present invention is low-cost, while military 3D radar systems are orders
of magnitude more expensive, because of their phased array antennas. The
present invention does not use expensive phased arrays but uses marine
radars and PC-based processing to achieve considerable cost reduction,
especially as compared to military systems.

[0016]The U.S. and Canada have conceived and are developing a
North-American Bird Strike Advisory System (NABSAS). This system will
monitor and provide information to users on bird activity and hazards (to
aircraft) at numerous sites throughout North America. It includes a
network of avian radars as part of its data sources, and bird heights as
well as bird ground tracks are desired. 3D avian radars in accordance
with the present invention will provide ideal sources of bird information
for this Advisory System.

[0017]It will be obvious to those skilled in the art that the same
improvements described herein are applicable to low-cost radars used in
other applications such as homeland security. Any radar with plot
extraction (i.e. detection) could use the apparatus and method described
herein to estimate height of detected targets. Examples of such radars
are described in US Patent Application Publication No. 2006/0238406
entitled "Low-cost, High-performance Radar Networks," which is
incorporated herein by reference.

OBJECTS OF THE INVENTION

[0018]It is an object of the present invention to provide improved
state-of-the-art avian radar systems that extend current 2D target
localization capabilities to 3D ones.

[0019]A primary object of the current invention is to provide an
affordable 3D avian radar system capable of localizing bird targets and
other targets in three dimensions (latitude, longitude, and height).

[0020]Another object of the current invention is to provide the means to
affordably upgrade existing 2D avian radar systems so that they can
localize bird targets in 3D.

[0021]A key object of the present invention is to provide the means of
producing significantly more accurate target height estimates, as
compared to conventional 2D avian radars, while not reducing volume
coverage.

[0022]Another object of the present invention is to provide the means of
producing significantly greater volume coverage, as compared to
conventional 2D avian radars employing dish antennas, while not reducing
the accuracy of target height estimates.

[0023]Yet another object of the present invention is to improve the
accuracy of target RCS estimates.

[0024]A final object the present invention is to provide a radar system
that enables a determination as to whether a bird and an aircraft are
likely to collide.

[0025]These and other objects of the invention will be apparent from the
drawings and descriptions included herein. It is to be noted that each
object of the invention is achieved by at least one embodiment of the
invention. However, it is not necessarily the case that every embodiment
of the invention meets every object of the invention as discussed herein.

[0032]In accordance with the present invention, the following general
radar system designs provide (to varying degrees) the desired features
listed above: [0033]1. A radar system whose antenna has multiple
stacked pencil beams, and that switches between them rapidly in time
(sequential lobing) [0034]2. A radar system employing a monopulse antenna
that receives on multiple stacked beams simultaneously [0035]3. A radar
system with a single pencil beam that slowly scans up and down in
elevation, while rotating rapidly in azimuth. Such a system will not get
simultaneous height coverage and estimation, but will get them over time.
[0036]4. Two single-beam radar systems operating side-by-side at
different fixed elevation angles.

[0037]For the present invention, height-finding antennas and techniques
are applied to avian radar systems in order to provide a means for
providing height information about detected bird targets for BASH and NRM
applications. The invention uses custom-designed antennas preferably
fitted to a COTS radar transceiver (although using a custom-built radar
transceiver to facilitate integration still falls in the spirit of this
invention); and novel radar signal and data processing algorithms to
estimate the height of detected bird targets.

[0038]A 3D radar system comprises, in accordance with the present
invention, an antenna provided with means for means for varying its
effective pointing direction in elevation, a radar transmitter
operatively connected to the antenna for generating a radar signal for
emission via the antenna, a radar receiver operatively connected to the
antenna, an azimuth scanner operatively coupled to the antenna for
rotating same about an axis, and a processor operatively connected to the
receiver, the processor being configured for detecting and localizing
airborne targets in azimuth and range, the processor being further
configured for estimating a height of each detected target height based
on relative amplitudes of echo returns as a function of elevation
pointing direction of the antenna.

[0039]A related method of determining the heights of airborne targets
comprises, in accordance with the present invention, (a) operating a
radar system to illuminate and detect the targets, the radar system
having at least one radar antenna, (b) during the operating of the radar
system, varying an antenna elevation pointing angle of the radar antenna,
and (c) estimating detected target heights in accordance with variation
in amplitude of echo returns as a function of antenna elevation pointing
angle.

[0040]A first form of the present invention utilizes a switched-beam
concept that has an antenna with at least two selectable radar beams
pointed at different elevation angles. Each beam is preferably a pencil
beam with all beams having the same or similar azimuth response. The
azimuth beamwidth need not equal the elevation beamwidth, as is the case
when conventional dishes are used; different applications will have
different preferred aspect ratios. Each beam preferably has reasonably
low worst sidelobes (typically -20 dB), and has even lower ones at zero
elevation (typically -25 dB or lower). The lowest beam is preferably
elevated enough that zero-elevation ground returns are in its low
sidelobes; and the lowest beam may be elevated even higher. The second
beam is elevated typically between 1/2 and 2 beamwidths above the lowest
one, and any other higher beams will have similar separation. A preferred
embodiment has 1° beamwidth in azimuth, 3° beamwidth in
elevation, and has the 2 beams elevated at 5° and 9°.

[0041]A desirable option is to have the actual elevation of the beams
adjustable mechanically when the radar is offline (e.g. by
tilting/rotating the antenna structure to desired setting and fixing it
in place). In the above example, beams at 5° and 9° could
be the nominal (flat) setting, but the structure could be tilted up (i.e.
adjusted) so that they are at say 10° and 14°. An
electrical control could be provided as well so that a radar operator
could effectuate this mechanical adjustment using a joystick, slider or
some other convenient software or hardware control interface.

[0042]A preferred embodiment of a switched-beam antenna in accordance with
the present invention is a reflector antenna with two or more vertically
stacked feed horns, each horn being a simple single-mode flared waveguide
type. Offset feed designs may be preferred for achieving lower sidelobes
(eliminating feed blockage).

[0043]The antenna preferably rotates continuously 360° in azimuth
at-least 24 revolutions per minute (RPM) while transmitting and
receiving. It may be desirable to have a selectable rotation rate. The
rotating antenna is typically mounted near ground level; it could be on
the roof of a trailer or a small building, or it could have its own
dedicated structure. Some sites may require the antenna to be raised to
10 feet or so above ground in order to clear nearby obstructions. The
rotating antenna is usually protected from (or immune to) the environment
(wind, rain, dirt, etc.); any protective measures should not
significantly distort beam patterns nor raise sidelobes above tolerable
levels. The rotating antenna boresight must be (mostly) unobstructed from
mechanical apparatus; some applications may tolerate an obstructed
azimuth sector.

[0044]A high-power switch, usually in the 2 kW to 60 kW range to match the
power provided by a COTS marine radar transceiver, rotates with the
antenna and switches between the beams for both the transmitted and the
received signals. The processor preferably controls the switch, and can
switch between beams on a per-pulse basis according to an arbitrary
programmed pattern. Switching preferably occurs during the dead time
between the longest-range return and the start of the next transmitted
pulse. A rotary joint with a slip ring connection provides a path for RF,
power for the switch, and switching control signals while the switch and
antenna rotate in azimuth. A wireless connection, a battery, and/or some
other state-of-the-art schemes, could alternatively provide RF, power
and/or control to the switch, thereby obviating the need for a
specialized rotary joint.

[0045]An alternative switched-beam implementation does not require a
rotating or a high-power switch. A low-power switch operates on only the
received (Rx) signals. Transmission occurs out of both beams (or out of a
third beam that covers both). RF is delivered to the beams via the sum
channel of a dual-channel rotary joint and a hybrid. The Rx signals from
both beams are delivered to the switch via the hybrid, the sum and
difference channels of the dual-channel rotary joint and another hybrid.
Somewhat poorer elevation discrimination will result, because
transmission will be through both beams.

[0046]A second form of the present invention is a monopulse system, which
is an alternative to a switched-beam one, with the likelihood of higher
system cost and complexity. A monopulse system transmits out of a single
beam on every pulse, and simultaneously receives signals from two
distinct beams on every pulse. Beam shape requirements are similar to the
switched-beam concept. A monopulse system needs two receive beams stacked
in elevation, and a transmit beam that is just wide enough to cover both
receive beams. Transmission occurs out of both beams (or out of a third
beam that covers both). RF is delivered to them via the sum channel of
dual-channel rotary joint and through a hybrid. Monopulse requires two
receive paths, each from the antenna through the sampling system. The
received signals from both beams are delivered to the receivers via the
hybrid, the sum and difference channels of the dual-channel rotary joint
and through another hybrid.

[0047]A third form of the present invention is the slow-elevation-scanning
system, which is another alternative to switched beam. A single beam is
slowly nodded up and down in elevation while it rapidly rotates in
azimuth (helical scan). Nodding could be mechanical or electronic.

[0048]Nodding is slow enough that targets remain within the beam for
several consecutive scans, long enough to form tracks. The apparatus must
be able to control nodding while rotating in azimuth. Elevation coverage
is not obtained instantaneously, but over periods of a few minutes. This
is the scheme's key disadvantage: It does not detect every bird, but gets
the hourly, daily, seasonal activity (in this respect, it is like a
weather radar). This scheme has some key advantages over the multi-beam
solutions. It is more flexible in the choice of coverage region (e.g.
could look between 5° and 10° during day, 10° and
20° at night, etc.). It is a much simpler increment to the
currently existing solutions: The antenna is a simple conventional dish,
no modifications to the receiver and sampling system are required, and
the changes to the processing are confined to the interpretation of the
track data. The processor needs to be kept informed of the azimuth (Az)
and elevation (El) positions (via signals from scanner). The processor
preferably controls elevation according to operator-set parameters.

[0049]A fourth form of the present invention, which is an alternative to
the switched-beam system, involves using two (or more) independent
single-beam avian radar systems operating side-by-side with their
respective antennas set at different fixed elevation angles. Each avian
radar detects and preferably tracks targets within its respective
coverage volume, using its own receiver and processor. Detections and/or
tracks from each radar are combined in a downstream fusion processor,
which estimates height for each target based on its relative echo
amplitudes from each of the radars.

[0050]Regardless of the form of the present invention, for a given target,
its height estimate is based on the ratio of amplitudes received from
each beam in the target's range-azimuth cell, at as close to the same
time as possible. Preferably, the height-estimation algorithms use
interpolation to determine precisely where in elevation such a ratio
would occur, thereby producing a better height estimate. The radar
processor detects targets in each beam using state-of-the-art detection
methods known to those skilled in the art and preferably tracks targets
as well, using state-of-the-art multi-target tracking algorithms known to
those skilled in the art such as those detection and tracking algorithms
described in U.S. patent application Ser. No. 11/110,436 Low-cost,
High-performance Radar Networks] which are included herein by reference.
A multi-target tracker is preferably included in the processor as it
facilitates target track association (across beams) and allows for
smoothing of the noisy per-detection height estimates using methods known
to those skilled in the art, thereby producing better height estimates.
Various methods known to those skilled in the art can be used for
displaying the height of detected targets to users, including: color,
intensity, and/or numerical displays indicating the height for each
target, as well as statistical displays such as histograms which
characterize height distribution for several or all targets.

[0051]A related advantage of having height information is that more
accurate estimates of target radar cross-section (RCS) are obtainable,
aiding the classification of targets. When the radar system knows both
the azimuth and elevation angles associated with a particular target,
then target amplitude can be directly converted to RCS using methods
known to those skilled in the art. If the system does not know where the
target is relative to the (elevation) center of beam, then the target
amplitude has an unknown beam attenuation factor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0052]FIG. 1 is a block diagram of a switched-beam avian height-finding
radar apparatus in accordance with the present invention.

[0053]FIG. 2 is a block diagram showing a low-power-switch version of the
switched-beam apparatus shown in FIG. 1.

[0054]FIG. 3 is a block diagram of a monopulse avian height-finding radar
apparatus in accordance with the present invention.

[0055]FIG. 4 is a block diagram of a slow-elevation-scanning avian
height-finding radar apparatus in accordance with the present invention.

[0056]FIG. 5 is a block diagram of a frequency-scanning switched-beam
avian height-finding radar apparatus in accordance with the present
invention.

[0057]FIG. 6 is a block diagram of a multiple, side-by-side avian
height-finding radar apparatus in accordance with the present invention.

[0058]FIG. 7 is a block diagram of the radar processor subsystems and
output destinations.

DETAILED DESCRIPTION

[0059]A block diagram of a switched-beam avian height-finding radar
apparatus 1 in accordance with the present invention is shown in FIG. 1.
Characteristics of each block are as follows. The avian height-finding
radar apparatus 1 includes a radar transmitter 2 that is typically
noncoherent and transmits pulses of constant width at a constant pulse
repetition frequency (PRF) at X-band or S-Band (or other bands). Radar
apparatus 1 typically has either a continuously rotating or
sector-scanning antenna 3. Antenna 3 is typically mounted near ground
level within (or near) the area to be monitored.

[0060]The azimuth scanner 4 rotates the antenna 3 continuously in azimuth
while the antenna 3 is transmitting and receiving. The circulator 5,
limiter 6 and receiver 7 are conventional radar components such as those
found in marine radar transceivers. The sampling system, 8 digitizes the
radar return video signal.

[0061]The switched-beam antenna 3 has (at least) 2 selectable radar beams
15 pointed at different elevation angles. The high-power switch 10
rotates with the antenna 3 and switches between the beams for both the
transmitted pulse and the received signals. The processor 11 controls the
switch. The rotary joint with a slip ring connection 12 provides a path
for RF, power for the switch, and controls switching while the switch and
antenna rotate in azimuth.

[0062]The switch control circuit 13 drives the switch 10 into its
respective states. It preferably extracts pulse transmission timing
information from the RF signal 14 (or from transmitter exciter signals).
It forms switch state signals after programmed delays from the sensed RF
signal, with delays and switching pattern designated by processor 11.
Preferably, the switch changes state every pulse causing the beams to
alternate in a pulse-to-pulse fashion.

[0063]An alternate switched-beam implementation 20 is shown in FIG. 2. The
low-power switch 16 does not rotate with the antenna 3 and operates on
the Rx signals only. Transmission occurs out of both beams 15. RF is
delivered to them via the sum channel of dual-channel rotary joint 19 and
through hybrid 17. The received signals from both beams are delivered to
the switch via the hybrid 17, the sum and difference channels of
dual-channel rotary joint 19 and through hybrid 18.

[0064]A monopulse avian height-finding radar apparatus 21 shown in FIG. 3
is an alternative to switched-beam ones. RF pulses are delivered to both
beams 15 via the sum channel of dual-channel rotary joint 19 and through
hybrid 17. The two receive paths (L and U) 22, each run from the antenna
3 through to the sampling system 8. The received signals from both beams
15 are delivered to the receivers via the hybrid 17, the sum and
difference channels of dual-channel rotary joint 19 and through hybrid
18.

[0065]The slow-elevation-scanning avian height-finding radar apparatus 24
shown in FIG. 4 is another alternative in accordance with the present
invention. The antenna 3 is simpler than the above designs, with only a
single beam. The Az-El scanner 23 moves the antenna 3 through its helical
scan. The Elevation Rotary Joint 25 and Azimuth Rotary Joint 12 allow RF
transmission while scanning in both dimensions.

[0066]Scan-to-Scan Elevation Switching is an alternative mode for a
switched-beam system. The antenna remains at one elevation setting for
one scan, is switched to the other for the next scan, and then back, etc.
This halves the revisit time for targets only visible in one beam,
meaning a reduction in tracking performance. This solution could be used
if a switched beam antenna was available, but switching takes too long to
apply it on alternate pulses (for example, in the case of a mechanical
switch). The processor would analyze the alternating variation in
amplitude over several scans in order to derive height for any track. The
tracker must be set to handle targets that are only detected in every
other scan, which will happen for those at heights not within both beams.
The system could also be configured to mimic slow elevation scanning,
i.e. spend several consecutive scans at one elevation setting, then
switching to the next, etc.

[0067]The frequency-scanning apparatus 26 shown in FIG. 5 is an
alternative switched-beam system, where tuning of the transceiver RF
(from pulse to pulse) scans the beam in elevation, giving continuously
selectable beam positions. This gives much flexibility in the operator's
control of elevation coverage. The apparatus employs a flat-panel
frequency-scanned phased-array antenna 27. Such an antenna delivers
phased-array performance without the need for phase shifters, at much
reduced cost. Lower sidelobes (than typical reflectors) can be achieved
by careful design of the aperture taper. The radar transmitter 2 and
receiver 7 must be rapidly tunable over a fairly wide bandwidth, which
prevents the apparatus from using inexpensive COTS marine radars.

[0068]An alternative height-finding avian radar system 28 shown in FIG. 6
consists of two (or more) side-by-side avian radars, where one radar
subsystem 29 operates at a lower elevation angle, the other radar
subsystem 30 at higher one. Each radar subsystem 29 and 30 has its own
receiver 7, sampling system 8 and processor 11. Tracks (or detections)
are combined in fusion processor 31, which then derives height estimates
for detected targets.

[0069]Other scanning alternatives are possible, but the above are more
suited to avian radars, where 360° azimuth coverage is usually
required. One could scan quickly mechanically up-and-down (or around) in
elevation while rotating slower in azimuth. One could scan in 2D in a
back-and-forth raster mode (electronic, mechanical, or both). While a
phased-array antenna could be integrated into the radar sensor of the
present invention, it is not a preferred embodiment of the present
invention due to the significantly higher cost anticipated for such an
antenna.

[0070]Preferably, embodiments of a radar system as disclosed herein aim to
take advantage of standardized COTS technologies to the maximum extent
possible in order to keep the system cost low and to provide for low life
cycle costs associated with maintainability, upgrade ability and
training. Preferably, COTS marine radars are used as the radar sensor in
order to minimize sensor costs. The radar processor 11 itself
incorporates sophisticated algorithms and software that runs on COTS
personal computers (PC). Preferred embodiments provide a low-cost,
high-performance, land-based radar sensor designed for avian radar
applications. Preferred embodiments digitize the raw radar video signal
from the marine radar receiver and use a PC-based radar processor with
sophisticated processing such as the detection, tracking and display
processing described in US Patent Application Publication No.
2006/0238406 entitled "Low-cost, High-performance Radar Networks," which
is incorporated herein by reference and further described below.

[0071]The radar processor 11 shown in FIG. 7 preferably incorporates a
detection processor 32, a track processor 33, a post-processor 34 and a
display processor 35. The detection processor 32 performs radar signal
processing functions known to those skilled in the art such as
scan-conversion, clutter suppression through the use of adaptive
clutter-map processing to remove ground and weather clutter, sector
blanking to suppress detections and interference in regions that are not
of interest, adaptive thresholding such as constant false alarm rate
(CFAR) processing, and digital sensitivity time control (STC). The
detection processor declares the presence and location of target plots 36
preferably on each radar scan. The information on each plot preferably
includes time, range, azimuth, elevation (beam center), and amplitude.
The track processor 33 sorts the time-series of detections (also called
plots) into either target tracks 37 (confirmed targets with estimated
dynamics) or false alarms. The information on each tracked target
preferably includes time and estimated 3D spatial position, velocity, and
RCS.

[0075]The height-finding algorithms in accordance with the present
invention, for a given target, are based on the ratio of amplitudes
received from each beam in the target's range-azimuth cell, at as close
to the same time as possible. Antenna calibration data (previously
acquired) are used to translate the target amplitude ratio to an estimate
of the target elevation angle, which can then be translated to a height
estimate through simple geometry. Preferably, the height-estimation
algorithms use interpolation to determine precisely where in elevation
such a ratio would occur, thereby producing a better height estimate.
Some nonlinear function of amplitude could also be used in place of
amplitude. The elevation beam pattern for each beam of the antenna needs
to be calibrated, or alternatively, the ratio itself. Any antenna
calibration method known-to-those skilled in the art may be used to
generate the required calibration data and table look-up methods known to
those skilled in the art may be used to directly provide height
estimates. The radar processor 11 detects targets in each beam using
state-of-the-art detection methods known to those skilled in the art and
preferably tracks targets as well, using state-of-the-art multi-target
tracking algorithms known to those skilled in the art such as those
detection and tracking algorithms described in US Patent Application
Publication No. 2006/0238406, which are included herein by reference.

[0076]A multi-target tracker, such as the aforementioned MHT/IMM automatic
multi-target tracker which is ideal for surveillance tracking with many
targets, is preferably included in the processor 11 as it facilitates
target track association (across beams) and allows for smoothing of the
noisy per-detection height estimates using methods known to those skilled
in the art, thereby producing better height estimates. Consider the case
where the antenna switches between two elevation beams every pulse. For
each full azimuth scan (revolution) of the antenna, two scan matrices of
radar echo data are produced, one for each of the two beams. Detections
are automatically computed for each of the scan matrices, and this
process is repeated continuously from scan to scan. Each detection
includes a location (e.g. range/azimuth) and an amplitude (or some
nonlinear function of amplitude). Without a multi-target tracker,
determining which detections from the first scan matrix are associated
with which detections in the second scan matrix (i.e. arise from the same
respective targets) is a very difficult task. This is so because
detections are inherently noisy and false alarms confuse the situation.
This is even more the situation when low detection thresholds are used to
improve detection sensitivity as is done in US Patent Application
Publication No. 2006/0238406. As a result, averaging the resulting
amplitudes (or ratios) over multiple scans does not perform as well as
one would hope due to incorrect associations. With a multi-target tracker
operating independently on each of the scan matrices over time,
high-quality confirmed tracks result. For each target, its track will
preferably record the amplitude from each detection used in the formation
of that track, and preferably smooth the sequence of amplitudes to form a
more accurate target amplitude estimate within that particular beam. Now
track-to-track association methods known to those skilled in the art can
be used across the beams to associate tracks resulting from the series of
first scan matrices with those resulting from the series of second scan
matrices that belong to the same respective targets.

[0077]Finally, the ratio of amplitudes can preferably be computed on a
scan-by-scan basis from the smoothed amplitude estimates from associated
track pairs in order to compute a series of height estimates that are
effectively smoothed over multiple scans, thereby resulting in more
robust and more accurate height estimates. Alternative smoothing
techniques are to smooth the per-scan height estimates or the per-scan
amplitude ratios, but these methods tend to be less robust to
interference and missed detections.

[0078]A related advantage of having good target height (or equivalently
elevation angle) information is that more accurate estimates of target
radar cross-section (RCS) are obtainable. RCS is a property of a target;
however, it is estimated using target echo amplitude. Target echo
amplitude is dependent on the two-way beam pattern, which can be
characterized as having a gain in the azimuth direction and a gain in the
elevation dimension. When the radar system knows both the azimuth and
elevation angles associated with a particular target as in the present
invention, then target amplitude can be directly converted to RCS using
radar equation and beam pattern calibration methods known to those
skilled in the art. If the system does not know where the target is
relative to the (elevation) center of beam, then the target amplitude has
an unknown beam gain factor, making a good target RCS estimate
impossible.

[0079]Good RCS estimates can lead to the ability to better classify
different classes of targets. For example, an eagle will have a larger
RCS than a sparrow. Improving the quality of RCS estimates will
ultimately improve one's ability to use these estimates along with other
radar descriminants to classify targets.

[0080]The processed information produced by radar processor can be
presented to the operator on a local real-time display. This information
may include scan-converted video, target data including detection data
(with time history) and track data, maps, user data (e.g. text, push
pins) etc. Preferred embodiments have radar target data geo-referenced
using a geographic information system (GIS) so that target data are
tagged to earth co-ordinates. Preferably, a map is integrated with the
radar display and provides a background on which is overlaid
geo-referenced radar data.

[0081]The track data produced by preferred embodiments contains detailed
(but compact) long-term behavior information on individual targets. For
any given scenario, these data can be automatically tested for hazardous
activity, in order to generate alerts. Because the information is
detailed, alerts can reflect complex behavior, such as origins and
destinations of birds, runway approaches, density, etc. Target detection,
tracking and hazard recognition algorithms may be customized for specific
hazards and scenarios. Alerts can include an audible alarm and display
indication to an operator, or a transmitted message to a remote user. The
low-bandwidth track and alert information can be easily sent to central
locations, and directly to end users, providing economical, effective
monitoring. Automated alerts may be sent to remote users who require
them. This enables the radar surveillance system to run unattended with
users alerted only when necessary. Furthermore, track displays can be
provided to remote users to give them a clear picture of the situation
when alerts arise. The system preferably exploits COTS communication
technology to provide such remote alerts and displays inexpensively.

[0082]Many of the aforementioned radar processor features as well as
features not mentioned above are described in the articles Low-cost Radar
Surveillance of Inland Waterways for Homeland Security Applications,
Weber, P et al., 2004 IEEE Radar Conference, Apr. 26-29, 2004,
Philadelphia, Pa., and Affordable Avian Radar Surveillance Systems for
Natural Resource Management and BASH Applications, Nohara, T J et al,
2005 IEEE International Radar Conference, May 9-12, 2005, Arlington, Va.
and US Patent Application Publication No. 2006/0238406, all of which are
incorporated herein by reference.

[0083]For avian radar applications, one radar system, or even several
independently operating radar systems are often not enough to provide a
high-performance, composite picture covering the area of interest. For
any single radar, there are gaps in coverage due to obstructions, and the
area covered may not be a wide enough. One or more radar sensor
apparatuses can be connected to a network to distribute their composite
information to remote users. Since the target data contain all of the
important target information (date, time, position including height in
accordance with the present invention, dynamics, plot size, intensity,
etc.), remote situational awareness is easily realized. Radar systems as
disclosed herein may be networked to a central monitoring station (CMS).
In that case, the CMS has a fusion/display processor that processes,
integrates (and/or fuses), displays and archives the data. In addition to
monitoring live radar data, the CMS also provides the capability to play
back past recorded radar data. Some of the performance improvements
achievable through integration and fusion of data from radar networks
include: [0084]Spatial diversity against target fluctuations in RCS
(necessary for small targets) [0085]Spatial diversity for shadowing due
to geographic obstructions

[0086]A recorder can store the target data including track data and
detection data. Target data can easily be stored continuously, 24/7,
without stressing the storage capacity of a COTS PC. These same data can
be distributed over a network. The stored data can subsequently be played
back through any computer running the radar processor software; it is not
necessary that it be connected to a radar apparatus. This feature is
useful for off-line analysis. Target data can be archived for longer-term
investigations. The recorder supports continuous writing of target data
directly to a database (as well as to other file formats). The database
can reside locally on the radar processor computer, on another computer
on the network, or on both. The database is used preferably for
post-processing, for interaction with external geographical information
systems (GIS) systems, for remote radar displays, for support for web
services, and for further research and development (e.g. to investigate
and develop target identification algorithms).

[0087]The applications towards which the present invention is directed
require further research and development (R&D) in order to increase and
establish knowledge concerning target behavior. This knowledge can be
used, for example, for automatic target identification. Off-line analysis
of target data can be used with ground-truth data to better understand
bird signatures, which could then be used to develop bird identification
algorithms. In BASH applications, knowing the kind of bird that is being
tracked is valuable for forming an appropriate response (e.g. should
aircraft delay take-offs and landings or make an evasive maneuver to
increase safety). Databases can continuously store complete target
detection and track data over extended periods of time in order to
support such R&D activities. One can rapidly play back stored target data
into the radar processor in order to study and analyze the data.

[0088]Particular features of our invention have been described herein.
However, simple variations and extensions known to those skilled in the
art are certainly within the scope and spirit of the present invention.
This includes variations on integration of the functional blocks
described herein. For example, the sampling system 8 could be integrated
with the processor 11 forming a single functional unit, without departing
from the spirit of the invention.